Current voice telephone system operators are generally engaging in a growth phase of deploying data services to their subscribers. One of the chief ways to do this is to offer digital subscriber lines (DSL) wherein both voice and data may be carried over a common twisted pair cable to a subscriber's residence or business. In one of the more popular versions of DSL, Asymmetric Digital Subscriber Line (ADSL), a twisted pair carries two types of duplexed signals over different frequency bands. The first signal is the voice signal, generally at 4 KHz and below. The second signal is the data signal, generally modulated at above 4 KHz.
Other forms of DSL may inter-mingle voice signals with data, by, e.g. voice over packet (VOP). Such forms include: Symmetric DSL (SDSL), High bit-rate DSL (HDSL), and Very-high-bit-rate DSL (VDSL). In such cases, the voice signal may be modified by a Integrated Access Device (IAD) to be converted into data packets. A IAD is intended to provide access to the twisted pair by multiplexing at least one voice signal with other signals, such as data, which is used most commonly by computers for internet access. Such a device, coupled with DSL service can provide a great improvement to home-owners or small businesses because the cost to maintain a single twisted pair subscriber line that multiplexes various signals is much less than having a dedicated subscriber line for each device at the customer premises.
Unfortunately, because the IAD is performing high level communications functions—essentially changing the signaling format between dissimilar networks—the IAD may be susceptible to power failures that leave none of the customer premises equipment operating. Although the data culture associated with computers has long accepted, in many situations, the possibility of intermittent failures, the opposite is true for the voice telephony world. Namely, in many parts of the world, a telephone is viewed as a necessity, and particularly, a valuable tool to avert disasters through timely call placement to emergency personnel, e.g. calling 911. For this reason, telephone switches are required to have an up time greater than 99.999% of the time.
Thus the dilemma: how to offer fast data throughput, including that supporting voice, and maintain constant fault-free voice operation to a customer while keeping the number of subscriber lines between customer premises and data aggregator to a minimum.
Part of the solution lies in the ability to adjust the operation of a remote device, by transmitting a minimalist signal from the local device that is failing—a last gasp, if you will. A signal may be the encoding of two or more values (sometimes voltages) that change over time on a medium. A packet may be a series of changing values or an arrangement of signals. A protocol may involve the exchange of one series of changes on a medium. A signal may occur when a voltage changes or where an established protocol requires a responsive reply, but none occurs. Under ordinary circumstances a master DSL master modem communicates with a slave DSL modem component of the IAD so that the slave DSL modem synchronizes or trains to signals provided by the master DSL. Any failure of the slave DSL modem to respond under this protocol may be taken as a signal that the slave DSL is in a fault mode.
A data aggregator may be a Digital Subscriber Line Access Multiplexer (DSLAM) having at least one master DSL modem. Constant voice access has been accomplished for some forms of DSL by providing a secondary twisted pair line between a data aggregator and an IAD. Companies such as Coppercom provide a IAD that upon detecting a power failure, would route at least one of the telephones at a customer premises to the secondary twisted pair line—thus bypassing the data network altogether. For obvious reasons, it is twice as costly to maintain the two pairs of cabling from the data aggregator such as a switch or a DSLAM to the subscriber than in the situation where a single twisted pair is used. Nevertheless, the highly fault-resistant voice central office is much more reliably available than its counterpart, the IAD. This is true because central office (CO) equipment is more capable of providing reliability because of economies of scale, particularly due to CO backup batteries and other redundant power sources. Thus, there is a need to extend plain old telephone service (POTS) to an IAD that has failed in that the IAD is unable to transmit data packets.